X-ray phototimer that is compensated for dark current

ABSTRACT

X-ray film density is controlled with a phototimer that uses a detector for producing an electric current which is proportional to the instantaneous intensity of the X-rays that penetrate the film. The detector current is integrated with respect to time. The exposure is ended when the integrator output changes by a certain amount in which case the output from the integrator signals the X-ray generator to terminate. At this time, the film which is also integrating the X-radiation should be properly exposed. Leakage current varies between detectors and ordinarily causes an error in the integrated signal and, hence, in the exposure time. A circuit is provided to compensate for whatever leakage current error would exist with the particular detector that is used.

United States Patent [72] Inventor [2i Appl. No. [22] Filed [45]Patented [73] Assignee [54] X-RAY PHOTOTIMER THAT IS COMPENSATED 12/1967Cashion et al. N;

Primary Examiner-Archie R. Borchelt Assistant Examiner-A. L. BirchAttorneys- Ralph G. Hohenfeldt, Joseph B. Forman, Frank L.

Neuhauser and Oscar B. Waddell ABSTRACT: X-ray film density iscontrolled with a phototimer that uses a detector for producing anelectric current which is proportional to the instantaneous intensity ofthe X-rays that penetrate the film. The detector current is integratedwith respect to time. The exposure is ended when the integrator outputchanges by a certain amount in which case the output from the integratorsignals the X-ray generator to terminate. At this time, the film whichis also integrating the X-radiation should be properly exposed. Leakagecurrent varies between detectors and ordinarily causes an error in theintegrated signal and, hence, in the exposure time. A circuit isprovided to compensate for whatever leakage current error would existwith the particular detector that is used.

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The output current from the photomultiplier is integrated with respectto X-ray exposure time with an operational amplifier integrator. Theintegrated output signal is fed to'a level sensor which may be acomparator that may be adjustably biased in accordance with the desiredtotal X-ray does for the proper exposure. When the integrated signalexceeds the bias signal, the sensor produces a signal which turns offthe Xray source indirectly.

Detectors produce undesirable stray leakage currents. A principal sourceof these errors is the photomultiplier tube that produces a small outputor leakage current (dark current) signal when voltage is applied to iteven though it is in the dark. This leakage current is added to theoutput signal which results from the tube being exposed to light. Itwould not be difficult to compensate for this leakage current if it werethe same with all tubes and all associated systems, but these quantitiesare variable. The practical approach to solving this problem has been touse photomultiplier tubes that are within a certain leakage currenttolerance and to reject those that are not. The result is that a premiumprice must be paid for photomultiplier tubes that are good enough forX-ray phototiming. Moreover, leakage current errors are reduced, but noteliminated by using only those tubes that are within the specifiedtolerance.

SUMMARY OF THE INVENTION An object of the present invention is toprovide an X-ray phototimer with an automatic compensator for a varietyof error causing signals associated with theuse of a photodetector. Darkcurrents due to electric leakage and thermionic emission in aphotomultiplier tube are compensated as are output currents due toambient room light leaking into the detector and reaching the tube.Electrical leakage in the phototube socket signal conductor andconnectors are also compensated. The small input bias current of theintegrator amplifier is compensated too.

Additional objects are to provide an automatic compensator that uses asmall number of components and that effects the correct amount ofcompensation regardless of the integrated X-ray dose that is chosen fora particular technical.

A preferred form of the invention supplies the photomultiplier tubecurrent to the inverting input terminal of an operational amplifier. Theamplifier acts as an augmented integrator since it has a resistor (foranticipation) in series with a capacitor in its feedback network. Thereis also a resistive-capacitive compensating network connected betweenthe one input and the output of the amplifier. Before an exposure isinitiated, the photodetector, such as a photomultiplier tube, isenergized and only leakage currents flow. The leakage currents cause avoltage to be developed and to become stable on a capacitor in thecompensating network. When the exposure is started, the feedbacknetwork, which has been short circuited until this time, is relieved ofits short circuit by operation of a relay and the integrator begins tointegrate the photocurrent. At the same time, the stored compensatingsignal voltage is applied to the integrator inverting input terminal tobring about what is effectively a subtraction of the leakage currentsignal from the total output signal of the photodetector.

How the foregoing and other specific objects are achieved will now beexplained in the ensuring description of an embodiment of the inventiontaken in conjunction with the drawmg.

DESCRIPTION OF THE DRAWING FIG. 1 is a block diagram of an X-rayphototiming system embodying the invention; and

FIG. 2 is a diagram showing the integrator and automatic leakagecompensator of the preceding figure in greater detail.

DESCRIPTION OF A PREFERRED EMBODIMENT In FIG. 1, the subject to beradiographed is given the reference numeral 1. During an examination,the subject is usually on an X-ray table top, not shown, beneath whichthere is a film cassette 2. Above the subject is an X-ray source 3. Raysof the proper intensity and duration penetrate the subject l and form animage on a film in cassette 2 in a known manner.

Means for setting the desired X-ray tube factors such as voltage, tubecurrent and a backup timer for the automatic exposure timer aresymbolized by a block 4 labeled X-ray power supply and control. Thisblock may include the usual high voltage transformer in whose secondarythere is a rectifier that provides unfiltered high voltage DC across theanode and cathode of the X-ray tube 3 during an X-ray exposure. Solidstate switches, not shown, may be included in the primary circuit of thetransformer for controlling energization of the X- ray tube. Thetransformer is usually turned on with an operator-controlled switch andtuned off by a timer when an exposure is completed. Without phototiming,the operator sets the control 4 for the X-ray tube voltage, current andconduction interval that will produce a film of desired density. Withphototiming the operator selects the voltage and current. The phototimerautomatically controls the product of current and conduction time interms of milliampere-seconds and it is this product that primarilydetermines film density or blackening for a given X-ray tube voltage anda given subject of examination. Low tube current for a long timeproduces the same film density as a high tube current for aproportionately shorter time. Phototiming is based on measuring totaldose to the film rather than the effect of any single tube factor.

In the FIG. 1 arrangement, a portion of the radiation which passesthrough subject 1 and cassette 2 is intercepted by a fluorescent screen5 which is inside a lighttight phototimer housing 6. FIG. 1 illustratesan exit detector. For an entrance detector, the positions of cassette 2and phototimer housing 6 would be interchanged. The brightness of thescreen 5 is proportional to X-ray intensity. Within housing 6 is aphotodetec tor 7 which produces an output signal current that isproportional to the brightness of screen 5 during an exposure. Thephotodetector may be an RCA 931A photomultiplier tube, for example.Other photomultiplier tube types may also be used. One embodiment of thepresent invention permits compensating this tube for leakage current ashigh as 50 nanoamperes in magnitude over exposure intervals ranging upto a maximum length of 3 seconds with an effective compensation error ofless than 1 nanoampere.

The photomultiplier tube, hereafter called PM7 for brevity, is energizedfrom a very stable power supply symbolized by block 8. The power supply8 adapted to energize PM7 before an X-ray exposure is started. PM7produces no leakage current until energized. In a practical case, powersupply 8 is turned on simultaneously with energizing the rotor, notshown, of the rotating anode X-ray tube 3 and a minimum of 0.7 secondsis allowed for the rotor to come up to full speed before suitableinterlocks permit an exposure to be initiated. Before exposure, leakagecurrent flows from PM7 after it is energized. The total output currentfrom PM7 is delivered by way of a conductor 9 to an integrator 10 whichis compensated for the leakage current resulting from PM7 with the newautomatic leakage compensator 11 which will be discussed in more detaillater. For the present, it is sufficient to observe that integratorintegrates output current from PM7 with respect to time and produces anoutput signal representative of the integral on its output line 12. Thissignal is introduced into a saturating operational amplifier or levelsensor 13. The level sensor is biased at a level that corresponds withthe desired amount of exposure. The variable bias circuit is shown inblock form and is marked 14. When the integrator 10 output signalexceeds the bias level, the exposure should terminate as a result of thelevel sensor 13 switching from one saturated state to another anddelivering an exposure terminating signal through a filter 15 to anX-ray termination circuit 16.

Termination circuit 16 acts on X-ray control 4 to cause the high voltageto be removed from the X-ray tube 3 which is synonymous with terminatingthe exposure. As is evident from the foregoing discussion, the outputsignal from integrator 10 should be the integral of photomultiplier PM7output current, less leakage current, with respect to time. AFter theintegrator output signal changes by a preset amount, the film incassette 2 which is also integrating should be properly exposed.

Attention is now invited to FIG. 2 which shows the circuitry forphotomultiplier tube PM7, integrator 10 and leakage compensator 11 forthe purpose of explaining how the leakage compensated integrated outputsignal is obtained.

FIG. 2 is a simplified schematic diagram of the circuitry showing itsstate during the delay period of about 0.7 seconds when the X-ray tuberotor is coming up to speed in expectation of making an exposure. Highvoltage is applied to PM7 at the same time that the rotor is energized,that is, at the beginning of the delay period, and leakage currentbegins to flow from PM7. It will be shown that the leakage current valueis stored or remembered and used to compensate the signal from PM7during an X-ray exposure.

An operational amplifier 17 has its augmented feedback circuit,comprising R1 in series with Cl, short circuited by a first normallyclosed contact K2A when leakage current begins to flow. Assume leakagecurrent to be negative, that is, the direction of conventional currentis from the amplifier input, to the phototube anode. This circuit willcompensate positive or negative leakages equally well; however, theactual leakages obtained in practice are usually negative. Capacitor C1charges rapidly to the indicated polarity if there is negative leakage.Storage capacitor C3 in the compensation loop is initially discharged sono current flows through high resistance R3 at the start. But, C3 beginsto charge positively, as indicated, through normally closed contact KlBand R2. The voltage on C3 follows the voltage on C2 but lags it becauseof the R2, C3 time constant. With a positive voltage across C3, part ofthe leakage current now flows through R3 and the current to C2decreases. The current to C2 decreases and the current through R3increases until at steady-state the current through R3 equals theleakage current and the current of C2 equals zero. At this time, C2 andC3 are both charged to a positive voltage equal to leakage current timesthe ohmic value of R3.

Steady-state is reached before the end of the delay period. At the endof the delay period, when the X-ray exposure is initiated, reed relaycoils, not shown, are energized in the proper sequence and the contactsin FIG. 2 change state. First, normally closed contact KlB opens so thatthe voltage across C3 is no longer affected by the output of amplifier17. Then the normally open contact KIA closes, short circuiting C2 andbring the amplifier 17 output to zero. A few milliseconds later,normally closed contact K2A opens to remove the short circuit fromacross series connected R1 and the integrated capacitor C1. The feedbackcircuit then comprises R1, C1 and closed contact KIA in series betweenthe output terminal of operational amplifier 17 and its inverting inputterminal. Thus, the integrator is ready to operate before an exposurebegins.

When an X-ray exposure begins, PM7 delivers a current that isproportional to the instantaneous brightness of fluorescent screen 5 inthe phototimer plus leakage current. However, compensating capacitor C3is now charged to the leakage current value so the leakage current fromPM7 will flow through R3 instead of the integrating capacitor circuit R1and C1. Since the R3, C3 time constant is chosen large compared to themaximum exposure length of about 3 seconds, most of the leakage currentwill continue to flow through R3 and not through the integratingcapacitor for the duration of the exposure.

A commercial embodiment of the solid-state phototimer is adapted tocorrect for leakage currents ranging up to a magnitude of 50 nanoamperes(na.) with a maximum error in compensation of 1.9 na. The error incorrection is due to slight discharging of C2 during an exposure. Forthis maximum 1.9 na. error, the error increases nearly linearly fromzero at the beginning of the exposure to 1.9 na. at the end of theexposure. Thus, the average or effective correction error is 0.95 na.This represents 3 percent error for a minimum PM signal current of 33na. at 3 seconds exposure length. For shorter exposures, the error isless than 3 percent. Thus, photodetectors with large dark currents maybe used rather than rejected. This circuit will correct leakages of morethan 50 nanoamperes, but in this case, the effective compensation errorwill be greater than 0.95 nanoamperes.

An illustrative embodiment of the circuitry in FIG. 2 uses RCA 931Aphotomultiplier tube. The operational amplifier may be any type that hasan input bias current which is only a small part of the leakagecorrection range. R3 is 40 megohms R2 is 5 kilohms (k.). R1 depends onthe frequency of the X- ray pulses and, hence, the frequency of theoutput pulses from PM7. For a standard SCR X-ray contactor: at 60 Hz.3-phase, R1 is 82 k.; at 60 Hz. l-phase, R1 is 27 k.; at 50 Hz. 3-phaseand l-phase, R1 may be respectively, k. and 33 k. Cl, c2 and C3 are 0.1,0.00] and 2.0 microfarads, respectively. Test results show thatcompensator response speed is adequate with these parameters as thecapacitor C3 voltage is within one-half percent of the steady-statewithin 0.16 seconds which is well within the 0.7 seconds alloted forsteady-state to be reached.

Although an illustrative embodiment has been described in some detail,it will be understood that the principles of the new automatic means forcompensating an X-ray phototimer for photodetector leakage can bevariously embodied. Therefore, the scope of the invention is to bedetermined only by construing the claims which follow.

1 claim:

1. For use in a diagnostic X-ray system that includes an X- ray sourceadapted to project an X-ray beam through an examination subject onto anX-ray image recording means, a compensated X-ray phototimer comprising:

a. detector means adapted to produce on its output terminal an electricoutput signal that is representative of the instantaneous intensity ofthe X-ray beam which has penetrated the examination subject, saiddetector means also producing a leakage current in the absence andpresence of X-radiation,

b. amplifier means having an output terminal and an inverting inputterminal, the latter of which is connected to the output terminal of thedetector means,

c. an integrating feedback network connected between the inverting inputterminal and output terminal of the amplifier means, said networkcomprising first and second capacitors connected in series between saidinput and output terminals and a first normally closed switch contactconnected in parallel with the first capacitor and a second normallyopen switch contact connected in parallel with the second capacitor,

d. a third leakage signal storage capacitor, a resistor and a thirdnormally closed switch all in series and connected jointly to saidamplifier output terminal and said second capacitor, and

switch being closed and said first and third switches being opened whenX-radiation is initiated, whereby to apply the voltage that is stored onthe third capacitor to the inverting amplifier input terminalsimultaneously with the total signal from the detector means to therebycompensate for leakage current after X-radiation is initiated.

1. For use in a diagnostic X-ray system that includes an X-ray sourceadapted to project an X-ray beam through an examination subject onto anX-ray image recording means, a compensated X-ray phototimer comprising:a. detector means adapted to produce on its output terminal an electricoutput signal that is representative of the instantaneous intensity ofthe X-ray beam which has penetrated the examination subject, saiddetector means also producing a leakage current in the absence andpresence of X-radiation, b. amplifier means having an output terminaland an inverting input terminal, the latter of which is connected to theoutput terminal of the detector means, c. an integrating feedbacknetwork connected between the inverting input terminal and outputterminal of the amplifier means, said network comprising first andsecond capacitors connected in series between said input and outputterminals and a first normally closed switch contact connected inparallel with the first capacitor and a second normally open switchcontact connected in parallel with the second capacitor, d. a thirdleakage signal storage capacitor, a resistor and a third normally closedswitch all in series and connected jointly to said amplifier outputterminal and said second capacitor, and e. a resistor connected betweensaid third storage capacitor and said inverting amplifier inputterminal, f. said first and third switches being closed to charge saidsecond capacitor and third storage capacitor to a voltage of onepolarity representative of leakage current value when only leakagecurrent is flowing from the detector means prior to initiation ofX-radiation, said second switch being closed and said first and thirdswitches being opened when X-radiation is initiated, whereby to applythe voltage that is stored on the third capacitor to the invertingamplifier input terminal simultaneously with the total signal from thedetector means to thereby compensate for leakage current afterX-radiation is initiated.